Process and device for forming raised metallised contacts

ABSTRACT

Process and device is available for forming a raised contact metallization18) on a connection surface (11) of a substrate (10) with the use of a wire bonding device with a bonding tool (26). First of all, a wire end section of a contact material wire (13), drawn from a nose-piece (27), is connected to the connection surface (11) with the application of pressure and temperature, and subsequently a separation of the wire end section (29), connected to the connection surface (11), from the remaining contact material wire (13) takes place. The process further includes these steps: 
     making a first connection between a free end of the wire end section (29) and the connection surface (11) for the formation of a first connection region (21), 
     making a second connection between a running end of the wire end section (29) and the connection surface (11) or a partial region of the wire end section (29) in such a way that between the first connection region (21) and the second connection region (22) a defined length of wire (19) is formed, and the connection regions (21, 22) form together with the length of wire (19) a contact material volume (25), 
     remelting of the contact material volume (25) formed on the connection surface (11) for the formation of the raised contact metallization (18).

FIELD OF THE INVENTION

The present invention relates to a process and a device in accordancewith the preamble of claims 1 and 4.

BACKGROUND OF THE INVENTION

In the known processes for forming raised contact metallizations, whichare also technically described as so-called "bumps", a fundamentaldifferentiation is to be made between mask-oriented processes andprocesses which are rather to be described as mechanical.

Processes are described as mask-oriented processes where the arrangementand geometric state of the raised contact metallizations are defined bysurface masks which only leave free the regions of connection surfacesof a substrate surface which is otherwise covered by the mask. Only thenin a second procedural step is the contact material applied to theconnection surfaces, which are technically also described as so-called"pads", for example by means of galvanic or chemical separationprocesses.

In all of the aforementioned, mask oriented processes the selectiveapplication of the contact material intended for forming the raisedcontact metallizations is achieved by masking the substrate surface. Toassemble the mask, usually photolithographic processes for structuring aphotosensitive lacquer or similar processes are required. In addition,after the formation of the raised contact metallizations the mask mustagain be removed from the substrate surface. In particular the processexpenditure associated with the formation of such a mask is onlyworthwhile with mass production of substrates with standardizedconnection-surface distribution.

In contrast, the aforementioned mechanical processes for forming raisedcontact metallizations have the advantage that a selective formation ofraised contact metallizations on the pads is possible without theprevious formation of an appropriate surface mask. With such processesthe wire connection technology is used which has been developed initself to make wire connections between connection surfaces of the samesubstrate or different substrates. In wire connection technology aprocess technically described as "ball-wedge-process" has becomegenerally accepted, where first of all a ball is formed on a wire endsection, drawn from the nose-piece of a bonding capillary, by means ofthermal loading of the wire end. This ball is connected to the firstconnection surface with deformation by means of the nose-piece of thebonding capillary. Subsequently, a wire loop is formed to overcome thedistance between the first connection surface and a second connectionsurface, and finally to make the connection between the wire end sectionand the second connection surface the wire end section is pressed withdeformation against the second connection surface with a connectionregion of a pressure surface of the capillary nose piece, and withsimultaneous separation of the partial wire section is connected to theconnection surface. This second making of a connection is technicallydescribed as "wedge".

The use of this method known in itself from wire connection technologyfor the formation of a raised contact metallizations on a substrateconnection surface is disclosed in DE- 32 09 242.

In the known process a wire ball formed with thermal loading at the freeend of a wire end section drawn from the capillary nose piece is pressedagainst a substrate connection surface and is connected thereto. Theseparation of the wire end section drawn from the capillary nose pieceto form the raised contact metallizations takes place by forming a weakpoint at the wire end section drawn from the capillary nose piece and bysubsequently pulling at the weak point.

The raised contact metallizations formed with the known process has asubstantially circular base because of the basic spherical shape of thewire end section drawn from the capillary nose-piece. This results in arelatively small overlap between the raised contact metallizations andthe usually rectangular surface of the connection surface. In addition,the volume of the raised contact metallizations which can be attainedwith the known process is limited by the size of the contact materialball which was previously formed at the wire end section. The ball sizeis in turn dependent on the wire diameter, with the result thatproceeding from a given wire diameter only raised contact metallizationswith a fixed volume can be attained.

The object of the present invention is to suggest a process and a devicefor forming a raised contact metallization on a substrate connectionsurface, which make possible an adaptation of the volume of the raisedcontact metallization to the geometry and size of the connectionsurface.

The object is achieved with a process having the features of thefollowing described invention.

In the process in accordance with the invention first of all a firstconnection is made between a free end of the wire end section of acontact material wire and the connection surface in order to form afirst connection region, and subsequently a second connection is madebetween a running end of the wire end section, which is connected to theremaining contact material wire, and the connection surface or a partialregion of the wire end section connected to the connection surface inorder to form a second connection region, in such a way that between thefirst and the second connection region a defined length of wire isformed and the connection regions form together with the length of wirea contact material volume, and finally there is a remelting of thecontact material volume formed on the connection surface in order toform the raised contact metallization.

Differing from the previously known process disclosed in DE 32 09 242,in addition to the connection regions of the wire end section, thecontact material volume available to form the raised contactmetallization is defined by the length of wire produced between theconnection regions. Because the material volume of the length of wiredepends on the length of the length of wire, the volume desired for theformation of the raised contact metallization can be adjusted easily byan appropriate length of the length of wire. This type of volumeadjustment is independent of the cross section of the contact metal wireused. It is also possible, by means of an appropriate course of thelength of wire, to adapt the latter to a particular extent to thesurface geometry of the connection surface.

In this respect, the second connection can be made just as the firstconnection directly with the connection surface or also with a partialregion of the wire end section which is already connected at one end tothe connection surface. The second alternative proves advantageousparticularly if the connection surface only has a relatively smallsurface.

In particular for the latter case it also proves advantageous if thesecond connection of the wire end section with the partial region of thewire end section which is already connected at one end to the connectionsurface is made in such a way that the partial region corresponds withthe first connection region. In this way both connection regions arearranged directly one on top of the other, with the result that anarrangement of the connection regions is created such that a largeamount of space is saved and such that the connection regions overlap,whereby--as in the aforementioned cases as well--the material volumerequired to form the raised contact metallizations is providedsubstantially by the length of wire formed between the connectionregions.

In certain cases, however, it can also prove to be advantageous if thesecond connection of the wire end section with the partial region of thewire end section which is already connected at one end to the connectionsurface is made in such a way that the partial region is arranged withclearance from the first connection region. In this respect, theclearance can even be selected such that the second connection region islocated outside the connection surface, whereby, nevertheless, a definedlength of wire is created. This can be of advantage in particular ifvery high contact metallizations are to be formed on very smallconnection surfaces.

In particular with connection regions which lie relatively close to eachother or also one on top of the other, a formation of the bonding toolas bonding capillary proves advantageous, the capillary being providedwith a capillary nose-piece which has at least one pressure surface forthe formation of a connection region at a wire end section drawn fromthe capillary nose-piece, whereby adjacent to the pressure surface awire accommodation area is formed which is used for the recessedaccommodation of the wire end section in the surface of the capillarynose-piece.

With the wire accommodation area an avoidance area is formed for thelength of wire, so that the length of wire can be formed in asubstantially undisturbed manner even when the connection regions lieclose to each other or one on top of the other.

To be able to form lengths of wire not only in a main orientation itproves advantageous if the capillary nose-piece is provided with anotherpressure surface, arranged opposite the first one, for the formation ofa connection region, and if a wire accommodation area is likewiseprovided adjacent to this additional pressure surface. In addition,however, the process in accordance with the invention can also becarried out with bonding tools designed as bonding wedges.

In the following the process for forming raised contact metallizationsand a suitable device for carrying out the process will be explained byway of example with reference to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the formation of a length of wire between two connectionregions of a contact metal wire for forming a contact material volumefor a raised contact metallization.

FIG. 2 shows a raised contact metallization formed by remelting thecontact material volume shown in FIG. 1.

FIG. 3 shows another example for forming a contact material volume on aconnection surface.

FIG. 4 shows another example for forming a contact material volume on aconnection surface.

FIGS. 5, 6 and 7 show the formation of the contact material volume,shown in FIG. 3, in three successive phases, while showing a devicewhich can be used in this respect.

FIGS. 8 and 9 show the formation of the contact material volume, shownin FIG. 4, in two successive phases, while showing a device which can beused in this respect.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a connection surface 11 arranged on a substrate 10 andprovided, for example, with a gold application, on which connectionsurface there is arranged a contact material volume 14 formed from awire end section 12 of a contact material wire 13. The substrate can bea chip or another arbitrary substrate. To carry out the processexplained in the following it is only essential to have a connectionsurface 11 on which the contact material volume 14 can be formed withthe use of a wire bonding process known in principle.

The contact material volume 14 consists of two connection regions 15, 16of the wire end section 12 and a length of wire 17 formed in the shapeof a loop between the connection regions 15, 16.

In the exemplary embodiment shown in FIG. 1 the connection surface 11has a surface of 250×60 μm, whereby the longitudinal side is shown inFIG. 1. The contact material wire 13 consists here of a lead/tin solderalloy with a high lead content, for example PbSn2, whereby the wire endsection 12 has a diameter of 40 μm.

From FIG. 1 it becomes clear that the contact material volume 14 issubstantially determined by the length of the length of wire 17 formedbetween the connection regions 15 and 16.

FIG. 2 shows a raised contact metallization 18 produced from the contactmaterial volume 14 (FIG. 1) by remelting. With the contact material wire13 used here a temperature of about 310° C. is required for theremelting process. Because of the adhesion of the molten contactmaterial to the wettable connection surface 11 and because of thesurface tension of the molten contact material volume 14, the contactmetallizations 18 is formed with a relatively greatly pronouncedmeniscus. The contact metallization 18 shown in FIG. 2 is suitable, forexample, for flip-chip contacting of the substrate which is providedwith it.

In FIGS. 3 and 4 further examples for the formation of lengths of wire19 and 20 between connection regions 21, 22 and 23, 24 on the connectionsurface 11 are shown.

FIGS. 5 to 7 show in chronological sequence the formation of the lengthof wire 19, the formation of which is complete between the connectionregions 21 and 22 and which is shown in FIG. 3. To form the length ofwire 19 or, a contact material volume 25 formed from the length of wire19 and the connection regions 21, 22, a bonding tool constructed asbonding capillary 26 is activated in the so called "wedge-wedge-mode"and in this respect carries out the temporally successive movementsindicated in FIGS. 5 to 6.

The bonding capillary 26 shown in FIG. 5 in a longitudinal section inthe region of a capillary nose-piece 27 has a wire guide channel 28extending in this exemplary embodiment coaxially to the longitudinalaxis of the bonding capillary 26. A wire end section 29 of the contactmaterial wire 13 which can be moved out of the wire guide channel 28 bymeans of a feed device which is not shown in more detail is drawn fromthe wire guide channel 28. At the opening of the wire guide channel 28two pressure surfaces 31, 32 arranged opposite each other are provided,each of which passes over into a wire accommodation area 33, 34 formedhere in the shape of a V-slot in cross section. In thelongitudinal-section view shown in FIG. 5 in each case one can only seethe guide bevel 36 and 37 extending perpendicular to the drawing planefrom a slot base 30 inclined downwards to a contact surface 35 of thecapillary nose piece 27.

For the formation of the first connection region 21 shown in FIG. 5 thewire end section 29 projecting from the capillary nose-piece 27 is movedby means of the bonding capillary 26 towards the connection surface 11in such a way that in the configuration shown in FIG. 5 by means ofpressure of the pressure surface 31 the wire end section 29 is deformedwith the formation of the connection region 21 and is connected to theconnection surface 11. In this respect, a temperature and/or ultrasoundloading is superimposed on the pressure in such a way as is known fromwire bonding techniques currently practised. In technical terms thefirst connection region 21 is also described as the first wedge.

FIG. 6 shows a phase of the travel of the bonding capillary 26, where,superimposed by a feed movement 38 of the contact material wire, it ismoved upwards and subsequently downwards again in the direction of theconnection surface 11 (double arrow 39). In this respect, the loopshaped length of wire 19 shown in FIG. 6 is formed.

In FIG. 6 it can be clearly recognized how the contact material wire 13,with formation of the length of wire 19, is partially placed into thewire accommodation area 34, whereby the substantially undisturbed loopshaped formation of the length of wire 19 becomes possible. In addition,by means of the wire accommodation area 34 the formation of the pressuresurface 32 which is relatively small compared with the contact surface35 of the capillary nose-piece 27 becomes possible in order to attainhigh pressure and deformation forces with relatively little contactpressure of the bonding capillary 26.

FIG. 7 shows the formation of the second connection region 22 or secondwedge, where the contact material wire 13, with deformation by means ofthe pressure surface 32, is connected to the first connection region 21or wedge. When the connection is being made or immediately after it hasbeen made, the contact material wire 30 is pulled (arrow 40) so that thecontact material wire 13 is broken at a separating point 41 indicated bya zigzag line and in this way the contact material volume 25 alreadyshown in FIG. 3 is formed.

FIGS. 8 and 9 show in two successive production phases the formation ofthe length of wire 20, the formation of which is complete between theconnection regions 23 and 24 and which is shown in FIG. 4. A bondingtool constructed here as bonding wedge 43 is used to form the length ofwire 20 on a contact material volume 42 formed from the length of wire20 and the connection regions 23, 24. The bonding wedge 43 differs fromthe bonding capillary shown in FIGS. 5 to 7 substantially by a wiresupply channel 45 arranged such that it extends inclined to a wedgeextension 44. In this respect, the bonding wedge 43 corresponds to thebonding tools which are conventionally used in so-called "ultrasoundbonding".

FIG. 8 shows the formation of the first connection region 23, where therelevant wire end is connected to the connection surface 11. After theformation of the first connection region 23 the bonding wedge 43 ismoved upwards (arrow 46) until the length of wire 20 required to formthe contact material volume 42 is produced. Subsequently, to form thesecond connection region 24 the bonding wedge 43 is again moveddownwards (arrow 47) towards the connection surface 11 in such a waythat the connection region 24 is formed on the connection region 23, asshown in FIG. 9. The length of wire 20 formed in this respect in theshape of a loop is applied to the side of the top region and the wedgeextension 44 of the bonding wedge 43 and is deflected slightly in adirection at right angles to the drawing plane so that, even with theuse of the conventional bonding wedge 43 shown, the formation of thecontact material volume 42 with the loop-shaped length of wire 20 ispossible in a relatively unhindered manner.

After the second connection region 24 has been completed a separation ofthe running end of the contact material wire 13 by means of a tensileforce 49 acting in the longitudinal direction of the contact materialwire 13 also takes place here, as clarified by a point of separation 48.

It is pointed out that both the formation of the contact material volume25 shown in FIG. 3 and the formation of the contact material volume 42shown in FIG. 4 are possible with both bonding tools shown in FIGS. 5 to7 or 8 and 9.

We claim:
 1. A process for forming a raised contact metallization on aconnecting surface of a substrate with the use of a wire bonding devicehaving a bonding tool, whereby by means of the bonding tool to form afirst connection region first of all a first connection is made betweena free end of a wire end section of a contact material wire drawn from anose piece and the connection surface, and subsequently to form a secondconnection region, a second connection is made between a running end ofthe wire end section connected to the remaining contact material and theconnection surface and a partial region of the wire end section alreadyconnected at one end to the connection surface, and finally a separationof the wire end section connected to the connection surface from theremaining contact material wire taken place, wherein between the firstconnection region and the second connection region a defined length ofwire is formed, and the connection regions form together with the lengthof wire a contact material volume which is remelted in order to form theraised contact metallization.
 2. Process according to claim 1,characterized in that the second connection of the wire end section (29)with the partial region of the wire end section (29) which is alreadyconnected at one end to the connection surface (11) is made in such away that the partial region corresponds with the first connection region(21).
 3. Process according to claim 1, characterized in that the secondconnection of the wire end section (29) with the partial region of thewire end section (29) which is already connected at one end to theconnection surface (11) is made in such a way that the partial region isarranged with clearance from the first connection region (21).
 4. Abonding tool for forming a raised contact metallization according toclaim 1, having a capillary nose-piece which, adjacent to a wire guidechannel, has at least one pressure surface for forming a connectionregion at a wire end section drawn from the wire guide channel, and awire accommodation area arranged adjacent to the pressure surface,wherein the wire accommodation is arranged separated from the opening ofthe wire guide channel by the pressure surface and is used for therecessed accommodation of a partial section of the wire end sectionguided around the pressure surface in a contact surface of the capillarynose-piece.
 5. Bonding tool according to claim 4, characterized in thatopposite the first pressure surface (32) a second pressure surface (31)is arranged which, correspondingly adjacent to the first pressuresurface (32), passes over into a wire accommodation area.